N. Tomozeiu

Electronic sputtering of thin SiO2 films by MeV heavy ions

Abstract The rate of removal of material from SiO2 as a result of heavy ion irradiation, with energies in which energy loss via excitation and ionization of the solid predominates, depends strongly on the stopping power and angle of incidence of the incoming ions. There appears to be a threshold stopping power for SiO2 of 500 eV/(1015 at/cm2) (or 3.5 keV/nm). This electronic sputter yield has been found to reach values as large as 104 atoms/incoming ion for 66 MeV Ag ions at an angle of incidence of 7° with the plane of the surface. Strikingly, the electronic sputter yield is very small for thin SiO2 layers of a thickness ⩽1 nm when grown on c-Si, but it is appreciable for such layers deposited on the insulator silicon nitride. The data are discussed in the light of existing models for electronic sputtering invoking also models for potential sputtering of SiO2 by low-energy, highly charged ions.

Structure of sputtered silicon suboxide single- and multi-layers

The microscopic structure of silicon-rich and oxygen-rich SiOx (0<x<2) layers are very different. Generally, the Random Mixing Model (RMM) is used to describe the oxygen-rich SiOx layer structure in terms of microdomains of high- and low-oxygen content, respectively. We have studied the dimensions of spatial inhomogeneities in a-SiO2/a-Si multilayer stacks obtained by sputter deposition of Si in an Ar–O2 mixture. By using stacks of very thin layers, we have fabricated spatially inhomogenous structures as a model for the RMM. All stacks have the same total thickness (256 nm) and the thickness/layer is from 128 nm down to 2 nm. The composition and spatial inhomogeneities in the stacks were investigated by ion beam analyses techniques (Rutherford backscattering spectrometry (RBS) and high resolution RBS) and electron paramagnetic resonance (EPR). Infrared spectroscopy (IR) was used to study the local atomic structure of the samples. The EPR measurements, using different values of the microwave power, revealed two types of uncharged dangling bond defects. Their density amounts to approximately 1020 cm−3. We are able to detect spatial inhomogeneities down to 2 nm. This value is a firm upper limit for the spatial extension of domains in an RMM material.

Study of the a-Si/a-SiO2 interface deposited by r.f. magnetron sputtering

Abstract Samples of a-Si/a-SiO2 have been deposited on (100) c-Si substrates by r.f. magnetron sputtering from a polycrystalline-silicon target in an Ar/O2 gas mixture. Samples were either multi-layered stacks of a-Si/a-SiO2 or consist of a 50 nm a-Si intermediate layer covered by a single thin a-SiO2 (between 1 and 12 nm) layer. Two different procedures are used for deposition: (a) the transition from a-Si to SiO2 is made by addition of oxygen while the plasma is kept uninterrupted; (b) the transition is made by interrupting the r.f. power and Ar flow, followed by pumping the chamber to high vacuum before starting the combined Ar/O2 flow until process pressure is reached. Then the plasma is re-ignited. The atomic neighbors of Si in the top layer are investigated by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR). We find that the interface region is best characterized by a SiOx layer with x gradually increasing from 0 to 2 when the distance to the surface increases from 0 up to 3 nm for procedure (b) and 6 nm for procedure (a). The large interface width is attributed to the 40 s necessary to obtain a steady state concentration of atomic oxygen in the plasma as monitored in-situ by optical emission spectroscopy (OES).

Ion beam induced desorption from thin films: SiO2 single layers and SiO2/Si multilayers

Abstract The occurrence of O2 molecular loss from the bulk of SiO2 single layers and SiO2/Si multilayers as a result of 50 MeV Cu9+ irradiation has been investigated. This process did not take place with a significant rate, if it occurs at all. Instead both Si and O are removed from the SiO2 surface region, releasing molecular O2. If an elemental Si layer is on top in a multilayer, removal of Si and O with an appreciable rate is not observed. The irradiation creates bubbles in the SiO2/Si multilayers, which contain O2. The distinct SiO2 sublayers remain chemically intact. The bubbles deteriorate the depth resolution in elastic recoil detection.